High-Voltage Measurements and Isolation

Overview

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.

There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask is whether the system will be safe. Making high-voltage measurements can be hazardous to the equipment, to the unit under test, and to you and your colleagues. To ensure that the system is safe, you should provide an insulation barrier, using isolated measurement devices, between the user and hazardous voltages.

Table of Contents

1. What Is Isolation?

Isolation is a means of physically and electrically separating two parts of a measurement device, and can be categorized into electrical and safety isolation. Electrical isolation pertains to eliminating ground paths between two electrical systems. By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high voltages and transient voltages from transmitting across its boundary to other electrical systems with which you can come in contact.

Incorporating isolation into a DAQ system has three primary functions: preventing ground loops, rejecting common-mode voltage, and providing safety.

Ground Loops
Ground loops are the most common source of noise in data acquisition applications. They occur when two connected terminals in a circuit are at different ground potentials, causing current to flow between the two points. The local ground of the system can be several volts above or below the ground of the nearest building, and nearby lightning strikes can cause the difference to rise to several hundreds or thousands of volts. This additional voltage itself can cause significant error in the measurement, but the current that causes it can couple voltages in nearby wires as well. These errors can appear as transients or periodic signals. For example, if a ground loop is formed with 60 Hz AC power lines, the unwanted AC signal appears as a periodic voltage error in the measurement.

When a ground loop exists, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, Vg, which exists between the signal source ground and the measurement system ground, as shown in Figure 1. This potential is generally not a DC level; therefore, the result is a noisy measurement system, often showing power-line frequency (60 Hz) components in the readings.

To avoid ground loops, ensure that there is only one ground reference in the measurement system, or use isolated measurement hardware. Using isolated hardware eliminates the path between the ground of the signal source and the measurement device, therefore preventing any current from flowing between multiple ground points.

Common-Mode Voltage
An ideal differential measurement system responds only to the potential difference between its two terminals, the (+) and (-) inputs. The differential voltage across the circuit pair is the desired signal, yet an unwanted signal can exist that is common to both sides of a differential circuit pair. This voltage is known as common-mode voltage. An ideal differential measurement system completely rejects, rather than measures, the common-mode voltage. Practical devices, however, have several limitations described by parameters such as common-mode voltage range and common-mode rejection ratio (CMRR), which limit this ability to reject the common-mode voltage.

The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. Violating this constraint results not only in measurement error, but also in possible damage to components on the board.

Common-mode rejection ratio describes the ability of a measurement system to reject common-mode voltages. Amplifiers with higher common-mode rejection ratios are more effective at rejecting common-mode voltages. The CMRR is defined as the logarithmic ratio of differential gain to common-mode gain.

CMRR (dB) = 20 log (Differential Gain/Common-Mode Gain). (Equation 1)

Common-mode voltage is shown graphically in Figure 2. In this circuit, CMRR in dB is measured as 20 log Vcm/Vout where V- = Vcm.

Figure 2. CMRR Measurement Circuit

In a non-isolated differential measurement system, an electrical path still exists in the circuit between input and output. Therefore, electrical characteristics of the amplifier limit the common-mode signal level that can be applied to the input. With the use of isolation amplifiers, the conductive electrical path is eliminated and the common-mode rejection ratio is dramatically increased.

2. Isolation Considerations

There are several terms to know when configuring an isolated system:

Installation Category: A grouping of operating parameters that describe the maximum transients that an electrical system can safely withstand. Installation categories are discussed in more detail later.

Working Voltage: The maximum operating voltage at which the system can be guaranteed to continuously safely operate without compromising the insulation barrier.

Test Voltage: The level of voltage to which the product is subjected during testing to ensure conformance.

Transient Voltage (Over-voltage): A brief electrical pulse or spike that can be seen in addition to the expected voltage level being measured.

Breakdown Voltage: The voltage at which the isolation barrier of a component breaks down. This voltage is much higher than the working voltage, and often times is higher than the transient voltage. A device cannot operate safely near this voltage for an extended period of time.

Isolation Types

Physical isolation is the most basic form of isolation, meaning that there is a physical barrier between two electrical systems. This can be in the form of insulation, an air gap, or any non-conductive path between two electrical systems. With pure physical isolation however, we imply that no signal transfer exists between electrical systems. When dealing with isolated measurement systems, you must have a transfer, or coupling, of energy across the isolation barrier.

There are three basic types of isolation that can be used in a data acquisition system:

Optical Isolation
Optical isolation is common in digital isolation systems. The media for transmitting the signal is light and the physical isolation barrier is typically an air gap. The light intensity is proportional to the measured signal. The light signal is transmitted across the isolation barrier and detected by a photoconductive element on the opposite side of the isolation barrier.

Figure 3. Optical Isolation

Electromagnetic Isolation
Electromagnetic isolation uses a transformer to couple a signal across an isolation barrier by generating an electromagnetic field proportional to the electrical signal. The field is created and detected by a pair of conductive coils. The physical barrier can be air or some other form of non-conductive barrier.

Figure 4. Transformer

Capacitive Isolation
Capacitive coupling is another form of isolation. An electromagnetic field changes the level of charge on the capacitor. This charge is detected across the barrier and is proportional to the level of the measured signal.

Figure 5. Capacitor

Isolation Topologies

It is important to understand the isolation topology of a device when configuring a measurement system. It can be categorized into electrical and safety isolation. This section discusses electrical isolation. For electrical isolation, parts of a device are separated with isolation barriers that separate the card into sections. Multiple buses service each section individually. The 3 types of isolation can be listed from basic (low level protection) to complete (high level protection) in the order of: Channel to Earth Ground, Bank, and Channel to Channel isolation.

Channel-to-Earth GroundChannels of the device and the device's Earth ground are electrically isolated from one another. Channel-to-Earth isolation is represented in the figure 6, below. Voltages of the isolated front end (Va-c ) are on the same bus; these voltages are not isolated from one another. Ve,d are on a separate bus and are isolated from the front end. This is the most fundamental type of isolation, this protection is covered by bank and channel-to-channel isolation. Normally, this isolation type is present on NI 9000 series modules and some SCC modules.

Note: For the following diagrams, the diagonal hash marks indicate the isolation barrier, this separates circuitry. The transformer symbols represent electromagnetic isolation used to couple a signal across the isolation barrier by generating an electromagnetic field proportional to the electrical signal.

Figure 6. Schematic of Channel-to-Earth Isolation.

Bank (Channel-to-Bus) IsolationChannels of a device are banked (grouped) together to share a single isolation amplifier. Figure 7 and 8 represent Bank isolation. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the non-isolated part of the measurement system can be large. In other words, individual channels are not isolated, but the channel groups are isolated from one another and earth ground. Bank1, Bank2, Vi, and Vj,k are on separate buses and isolated from one another. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.

Figure 7. Schematic of Bank (Channel-to-Bus) Isolation.

Figure 8. Bank Topology

Channel-to-Channel
The most robust isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other non-isolated system components. In addition, each channel has its own isolated power supply. Figure 9 represents channel-to-channel isolation. Va,b, Vc,d, Ve,f, and Vg,h are all on separate buses and are isolated from one another.

Figure 9. Schematic of Channel-to-Channel Isolation.

In terms of speed, there are several architectures from which to choose. Using an isolation amplifier with an analog to digital converter (ADC) per channel is typically faster because you can access all of the channels in parallel. A more cost-effective, but slower architecture, involves multiplexing each isolated input channel into a single ADC.

Another method of providing channel-to-channel isolation is to use a common isolated power supply for all of the channels. In this case, the common-mode range of the amplifiers is limited to the supply rails of that power supply, unless front-end attenuators are used.

Figure 10. Channel-to-Channel Multiplexed Topology

Safety and Environmental Standards

When configuring a DAQ system, you must take the following steps to ensure that the product meets applicable safety standards:

Consider the operational environment, which includes the working isolation voltage and installation category.

Choose the method of isolation in the design based on these operational and safety parameters.

Choose the type of isolation based on the accuracy needed, the desired frequency range, the working isolation voltage, and the ability of the isolating components to withstand transient voltages.

Not all isolation barriers are suitable for safety isolation. Even though measurement products may have components rated with high-voltage isolation barriers, the overall product design, not just the components, dictates whether the device meets high-voltage safety standards. Safety standards have specific requirements for isolating humans from contact with hazardous voltages. These requirements vary among different applications and working voltage levels, but often specify two layers of protection between hazardous voltages and human-accessible circuits or parts.

In addition, the standards for test and measurement equipment are not only concerned with dangerous voltage levels and shock hazards, but also with environmental conditions, accessibility, fire hazards, and valid documentation for explaining the use of equipment in preventing these hazards. They maintain specific construction requirements of isolation equipment to ensure that the integrity of the isolation barrier is maintained with changes in temperature, humidity, aging, and variations in manufacturing processes.

When dealing with safety standards, the European Commission and Underwriters Laboratories, Inc. (UL) have outlined the standards that cover the design of high-voltage instruments. There are approximately 200 individual safety standards harmonized (approved for use to demonstrate compliance) to the Low Voltage Directive, which was the initial document that outlined the specifications for the voltage levels that require safety consideration.

The relevant standard for instrument manufacturers is EN 61010 -- Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use. EN 61010 states that 30 Vrms or 60 VDC are dangerous voltages. In addition to high-voltage design requirements, EN 61010 also includes other safety design constraints (such as flammability and heat). Instrument manufacturers must meet all the specifications in EN 61010 to receive the CE label.

There are two other standards very similar to EN 61010 -- IEC 1010 and UL 3111. IEC 1010, which was established by the International Electrotechnical Commission, is the precursor to EN 61010. The European Commission adopted it and renamed it EN 61010. UL 3111 is also a child of IEC 1010. UL took IEC 1010, made some modifications and adopted it as UL 3111. This new, strict UL standard replaces the older, more lenient UL 1244 standard for measurement, control, and laboratory instruments. For new designs, instrument manufacturers must meet all of the specifications in UL 3111 to receive a UL listing.

Installation Categories
The IEC defined the term Installation Category (sometimes referred to as Over-voltage Category) to address transient voltages. When working with transient voltages, there is a level of damping that applies to each category. This damping reduces the transient voltages (over-voltages) that are present in the system. As you move closer to power outlets and away from high-voltage transmission lines, the amount of damping in the system increases.

The IEC has created four categories to partition circuits with different levels of over-voltage transient conditions.

3. Typical Applications Requiring Isolation

Single-Phase AC Monitoring
To measure power consumption with 120/240 VAC power measurements, you record instantaneous voltage and current values. The final measurement, however, may not be instantaneous power, but average power over a period of time or cost information for the energy consumed. By making voltage and current measurements, software can make power measurements or do other analyses. To make high-voltage measurements you need some type of voltage attenuator to adjust the range of the signal to the input range of the measurement device. Current measurements require a precision resistor. The voltage drop across the resistor is measured, and Ohm’s Law (I = V/R) produces a current value.

Fuel Cell Measurement
Fuel cell test systems make a variety of measurements that require signal conditioning before the raw signal is digitized by the data acquisition system. An important feature for the testing of fuel cell stacks is isolation. Each individual cell can generate about 1 V, and a stack of cells can produce several kV. To accurately measure the voltage of a single 1 V cell in a large fuel cell stack requires a large common-mode range and high common-mode rejection ratio. Because adjacent cells have a similar common-mode voltage, bank isolation is sometimes acceptable.

High Common-Mode Thermocouple Measurement
Some thermocouple measurements involve high common-mode voltages. Typical applications include measuring temperature while a thermocouple is attached to a motor, or measuring the temperature dissipation capabilities in a conductive coil. In these cases, you are trying to measure small, millivolt changes with several volts of common-mode voltage. It is therefore important to use an isolated measurement system with good common-mode rejection specifications.

Serial Communication
Reliability is a number one concern when designing equipment to be resistant to the interference inherent in a harsh environment. Commercial and industrial applications such as such as POS networks, ATMs, bank teller stations, and CNC-based production lines are acceptable to voltage spikes and noise. Isolation reduces the possibility of damaging control systems and ensure that systems can maintain operational. Other applications that may require isolation are industrial process control, factory automation, serial networking devices, high-speed modems, monitoring equipment, long distance communication devices, printers, and remote serial device control.

4. National Instruments Isolated Products

NI offers many products that provide isolation for the measurement and automation application. Offerings include SCXI, SCC, and FieldPoint product lines, as well as high-voltage relays, switches, and DMMs. Most of the isolation products that NI makes fall under the standards outlined by IEC 1010-1 and UL 3111-1, which address standards for measurement, control, and laboratory use.

SC Express for Synchronized Isolated Measurements

SC Express is high performance conditioned measurements on the PXI Express platform. The architecture combines data acquisition and signal conditioning into a single PXI Express card to achieve best in class accuracy and performance. SC Express is ideal for high channel count, high performance applications especially where synchronization and data throughput are of upmost importance.

The PXIe-4300 is an 8 channel voltage input module with 300 Vrms CAT II channel-to-channel isolation and 3300 Vrms channel-to-earth 5 second withstand. Each channel has its own 16-bit ADC for simultaneous sampling at rates of up to 250kS/s per channel. Dedicated ADC's and throughput on the PXI Express bus ensure that adding channels does not reduce your sampling rate. The TB-4300 and TB-4300B are front-mounting terminal blocks with screw terminal connectivity. Choose the TB-4300 for ± 10V and the TB-4300B for ±300V input ranges. Programmable gains ensure maximum resolution from the ADC for each input range.

SCXI

Signal Conditioning eXtensions for Instrumentation (SCXI) is a signal conditioning and DAQ system for PC-based instrumentation applications. An SCXI system consists of a shielded chassis that houses a combination of signal conditioning input and output modules, which perform a variety of signal conditioning functions. You can connect many different types of sensors and signals directly to SCXI modules. The SCXI system operates as a front-end signal conditioning system for PC plug-in or PCMCIA DAQ devices.

Figure 12. SCXI Signal Conditioning and Data Acquisition System

The SCXI-1125 is an 8-channel isolated analog input module. The SCXI-1125 is CE certified as double insulated, Category II, providing 300 Vrms of working isolation between channels and channels-to-ground. The module also features programmable gains and lowpass filtering on each analog input channel, and you can expand the input range to 1,000 VDC with the TBX-1316 terminal block. This architecture is ideal for amplification and isolation of millivolt sources, volt sources, 0 to 20 mA, and thermocouples.

The SCXI-1121 and SCXI-1122 are designed for a wide variety of sensor and signal inputs requiring isolation. The SCXI-1121 offers independently configurable isolation amplifiers, filters, and excitation sources for each channel. The SCXI-1122 is a multiplexed module with a single isolation amplifier, filter, and excitation source for all channels. Both modules offer 250 Vrms working isolation and support strain, RTD, thermocouple, millivolt, volt, and 0 to 20 mA current input signals.

SCC Isolated Products

National Instruments SCC is a portable, modular signal conditioning system for use with M Series, E Series, and low-cost DAQ devices. SCC products condition a variety of analog input and digital I/O signals. With this modular design, you choose conditioning on a per-channel basis.

Figure 13. SCC Signal Conditioning

The SCC-A10 is a dual-channel module that accepts input voltage sources up to 100 V. Each channel of the SCC-A10 includes a 10x attenuation circuit and differential instrumentation amplifier with low-impedance outputs for maximum scanning rates by the DAQ device. The attenuation circuit includes high-impedance bias resistors, so you can connect floating or ground-referenced inputs to the SCC-A10 without adding external bias resistors. The SCC-A10 also provides over-voltage protection (up to 250 Vrms) for the DAQ system. SCC-AI analog input modules provide 300 V of Category II isolation. Each module provides lowpass filtering and input ranges from ±50 mV to ±42 V.

Plug-In Isolation Products

The NI Industrial M Series and SC Series PCI and PXI DAQ devices are designed for high-accuracy isolated measurements. Specialized terminal blocks allow for easy connectivity to external signals.

The SC Series PXI-4224 offers 60 VDC continuous channel-to-channel isolation on its 8 analog inputs. It has a +/-10V input range, 16-bit resolution, and a maximum aggregate sampling rate of 200kS/s with 30kHz of bandwidth per channel.

C Series Isolation Products

National Instruments offers C Series I/O modules for a variety of platforms and connectivity options with PC, industrial PXI, and embedded controllers. Several modules are compatible with USB carriers and USB NI CompactDAQ chassis for PC or PXI controlled operation with the NI-DAQmx driver. Additionally, C Series modules can be used in NI CompactRIO chassis for reconfigurable control and acquisition systems. Many of these modules are isolated from the digital communication lines with their controller, while some of them specialize in high-voltage isolation.

Figure 16. C Series Isolated DAQ

The NI 9206 is a 16-channel, bank isolated analog input module with 600VDC (US) channel-to-earth isolation. It has an input voltage range of +/- 10V with 16 bits of precision and a maximum aggregate sampling rate of 250 kS/s.

FieldPoint for High-Voltage Measurements

NI FieldPoint and Compact FieldPoint are modular, embedded control and distributed I/O systems for measurement, control, and data-logging applications that demand industrial-grade hardware with easy installation and configuration. Both FieldPoint and Compact FieldPoint feature built-in signal conditioning for direct connectivity to sensors and actuators. Modules are available for connecting to thermocouples, RTDs, strain gages, 4-20 mA signals, high-voltage sources, and many other signals. The FieldPoint and Compact FieldPoint products support both embedded control, by running NI LabVIEW Real-Time on a dedicated embedded processor, and connectivity to a PC over a variety of industrial buses (Ethernet, serial, CAN, Foundation Fieldbus). Compact FieldPoint products are designed to operate in harsh environments with electromagnetic noise, wide temperature ranges, and high shock and vibration.

FieldPoint and Compact FieldPoint product lines can measure and switch high-voltage signals. All the modules are isolated from the backplane with 250 V working isolation and 2300 V transient over-voltage protection. The cFP-AI-102 is a 12-bit analog input module capable of measuring +/- 120 VDC. The cFP-DI-330 is a digital input module for up to 250 V AC or DC, and the cFP-RLY-420 and FP-RLY-422 are relay modules capable of switching up to 250 VAC and 120 VDC.

Figure 17. Compact FieldPoint Industrial Control and Measurement

Isolated RS232 and RS485 Serial Communication

NI offers PCI, PXI, and AT serial hardware with up to 2000 V of optical isolation for safe, reliable communication in industrial environments. Isolated ports provide reliable communication in situations involving ground loops from different ground levels or high common-mode voltage induced on the lines in noisy environments. The isolation between each communication port and the host PC ensures the safe operation of the PC and the devices connected to other ports on the same board during accidental high voltages or line surges on communication lines.